This IMO is definatly untrue, or rather rumourness, why must I say this? Well three things.
1. TGS isn't even planned until Augustness or Julyness which is this month, and even so, they are done privatly.
2. The lists arn't even said from the company and Square-Enix loves being secretive, so even things that are said to appear at TGS, Square doesn't announce till last.
3. Nomura is already directing a FFVersusXIII game, and character designs for various other games, Directing KH3, and god knows how much he's needed for the FF7 Compilation Crisis Core, therefore, until he has completed those, it'd practically be impossible, no matter how godly at multitaskng it is, to even bring up a matter of Kingdom Hearts 3 to direct a storyline, let alone already have enough of a storyline to create a trailer with it, unless it's a really horrid graphic trialer.
All I can see shown at TGS is the FFXIII series, FFVII and probably some other FF stuff, or whatever else it is Square-Enix is making. Although if KH3 does appear (Which I highly doubt) till I see "Square's" List, or a Famitsu Magazine of TGS, there is no way it's humanly possible, or unhumanly possible in Nomura's case.
Then again this is all my own opinion, be free to counter it.